The present invention relates to an engine starting apparatus including an electric motor that produces auxiliary driving force used for assisting in cranking an engine.
For example, Japanese Laid-Open Patent Publication No. 6-62553 discloses such an engine starting apparatus that causes an electric generator such as an alternator to function as an electric motor, thereby assisting in cranking an engine. In this configuration, the generator motor is energized to assist in cranking the engine. When the rotation speed of the engine reaches a predetermined value, the engine starts igniting and rotating by itself. Switching between energization and stop of energization of the generator motor is generally executed by controlling an inverter having a switching element.
When a non-rotating generator motor is energized, a current (stator current), which is represented by the following equation (1), flows through the stator of the generator motor.Stator current=power supply voltage/(stator resistance+wiring resistance)  (1)
The wiring resistance in the equation (1) refers to the resistance of parts between the power supply and the generator motor other than the stator.
In the switching element, collector dissipation that is in proportion to the squared stator current is produced. The collector dissipation generates Joule heat, which increases the temperature of the switching element. On the other hand, the rotational speed and the current characteristics have the following relationship in the generator motor. That is, the stator current is large when the rotational speed is low and is decreased as the rotation speed is increased. Thus, when starting the non-operating engine, if the rotational speed of the generator motor is low, a large stator current is generated, which significantly increases the temperature of the switching element.
If such a state continues in which a large stator current flows during starting of the engine in a case where a general-purpose switching element having a relatively low heat resistance is used, the temperature of the element can exceed the rated temperature of the element. This state may occur, for example, when the engine friction is great and the generator motor does not rotate (locked) during starting of the engine. In general, energization stopping function is activated in which energization of the generator motor is stopped if the generator motor does not start operating for a predetermined period (for example, several tens of milliseconds) from the start of energization. This function is referred to as a lock protection function. Until the function is activated, a large stator current continues flowing.
To avoid such drawbacks, switching elements of a low on resistance or an increased thermal capacity have been proposed. A low on resistance switching element is configured to reduce the internal resistance so as to permit a large current to pass through. An increased thermal capacity switching element withstands a high temperature.
However, since low on resistance switching elements and increased thermal capacity switching elements are newly developed and thus costly, the cost of engine starting apparatus is significantly increased. Therefore, there is a demand for an engine starting apparatus that eliminates the above described drawbacks without using such expensive switching elements.